1. Technical Field
This invention relates generally to techniques for processing pulse signals and in particular to the analysis and discrimination of pulse signals in a nuclear spectroscopy system, such as those generated by a scintillation detector/photomultiplier.
2. Description of Related Art
The characteristics of geological formations are of significant interest in the exploration for, production and monitoring of subsurface water and mineral deposits, such as oil and gas. A variety of techniques have been developed to measure subsurface characteristics and evaluate the obtained data to determine the petrophysical properties of interest. These techniques typically employ the subsurface deployment of tools equipped with sources adapted to emit energy into the formations (typically through a borehole traversing the formations). The emitted energy interacts with the surrounding formations to produce signals that are detected and measured by one or more sensors or detectors on the tool. By processing the detected signal data, a profile or log of the subsurface parameters is obtained.
Of the many well logging instruments and techniques developed over the years to determine the hydrocarbon content and productivity of earth formations, the spectroscopy tool, by which energy spectra of the constituents of formation matrices and fluids are generated, has been found to provide information of particular value in formation analysis. In nuclear spectroscopy applications, the energy of incident nuclear particles is measured. In many cases this measurement is accomplished by measuring the energy deposited by the particle in a nuclear detector.
Parameters of subsurface formations which may be determined as a result of nuclear phenomena measurements include formation density, fractional volume of void or pore space in the formation (porosity), carbon/oxygen (C/O) ratios, formation lithology, and neutron capture cross section (Sigma), among other measurements. Parameters which may be determined by spectral analysis of the gamma-rays include concentration of various chemical elements, for example. Properties of fluids in the wellbore may also be determined from various gamma-ray measurements.
Gamma-ray scattering has been used for the determination of subsurface parameters in well logging for many years. Until recently, almost all of these measurements relied on the use of chemical sources of gamma-rays (mainly 137CS) or X-ray tubes operating continuously. Gamma-ray sources pose a significant radiation hazard for handling and storage. In addition, these sources are under scrutiny as they could potentially be used in the manufacture of a dirty bomb.
Processing of signals from nuclear sensors has a long history and many approaches are well documented in the open literature (See e.g., the book by GLENN KNOLL, RADIATION DETECTION AND MEASUREMENT (3rd ed., John Wiley 1999)). Most data acquisition in low energy nuclear physics deals with continuous radiation at count rates, which make it possible to distinguish and analyze the signals from single gamma-rays with a detector.
In the early 1980s tests were performed with a downhole linear accelerator. This device was capable of producing a large flux of high energy x-rays. However, the production occurred in very short bursts of very high intensity. Experiments have been conducted with other downhole pulsed x-ray devices. In most cases, it has not been possible to measure single x-ray events and to perform x-ray spectroscopy. In these cases the traditional pulse processing schemes do not work, since multiple x-rays become convolved in a single electronic pulse. This reduces or eliminates spectroscopy information contained in the signal.
FIG. 1 shows a comparison of the time sequence of x-ray signals as seen at the output of a detector with a continuous source of x-rays and with a pulsed source of low duty cycle. The top left of the figure shows the sequence of pulses arriving randomly in the detector at an average of about 14,000 counts/second. The top right shows the time sequence if the average count rate is increased by a factor 10. The bottom reflects the same total x-ray flux in the same detector using a pulsed source with a duty cycle of 0.2%, i.e., the source emits x-rays during 1 μs out of 500 μs. The vertical scales differ by a factor 10 between the top and the bottom graphs. The signal at the output represents the total of the pulses in a 1 μs interval equivalent to the integral of the pulses in a 500 μs interval with a continuous source of the same average output.
At very high instantaneous count rates the scattered gamma-rays or x-rays arrive at the detector quasi-simultaneously. The information from the pulses is summed and there is no spectroscopy information left from indistinguishable single pulses. The only information left is the sum total of all the pulse heights registered during the interval of the x-ray burst.
A need remains for improved techniques to process signals from nuclear sensors when pulsed source devices are used.